1. Field
The present invention relates to propylene polymers and to processes for producing propylene polymers, particularly propylene-ethylene copolymers. These copolymers are prepared using metallocene catalysts.
2. Description
Propylene homopolymers and copolymers (“propylene polymers”) are used in a wide variety of applications including fibers, films, adhesives and molded articles. These polymers may be amorphous, elastic or have varying degrees of crystallinity. They may be used alone or blended with various other olefin polymers to obtain particular properties. Examples include block copolymers, random copolymers, impact copolymers and thermoplastic polyolefins.
Various processes have been proposed for making propylene polymers. These processes typically employ different variables and parameters, including different monomer compositions, solvents, additives, reaction conditions, catalyst systems, etc. The properties and characteristics of the final product have a great deal to do with selected process variables and parameters, particularly with respect to the particular catalyst used.
The use of metallocene catalysts to make propylene polymers is well known. Metallocene catalyzed products offer certain advantages over Ziegler-Natta catalyzed products, primarily as a result of more uniform composition and molecular weight distributions. It is well known that bridged or stereorigid, chiral metallocenes are required for making high tacticity propylene polymers.
The use of metallocenes for preparing high tacticity propylene polymers has been very well studied. Investigations into these metallocenes first focused on the specific type of bridge between the two cyclopentadienyl or indenyl groups. It was found that silicon-based bridges, as opposed to carbon-based bridges, provided much higher molecular weight and melting point. See, e.g., EP 284 708 B1, U.S. Pat. Nos. 5,846,896, and 5,017,714. Once this phenomenon was well established, investigations focused on varying particular indenyl substitutions as a means of further increasing molecular weight and melting point. After this discovery, there was apparently little or no further investigations into propylene polymers prepared with carbon or alkylene radical bridged metallocenes.
Another early learning was that copolymers produced with metallocene catalyst had a far more efficient and uniform comonomer incorporation compared to copolymers prepared with traditional Ziegler-Natta catalysts, which yield copolymers having a disproportionate amount of comonomer concentrated in the lower molecular weight portion of the product. See, e.g., EP 318 049 B1 and EP 374 695.
It is also now well established that molecular weight, crystallinity, and melting point decrease substantially as comonomer content increases. This phenomenon has been exploited, for example, to improve the processability of nonwoven fabric made from propylene copolymers. See, e.g., U.S. Pat. Nos. 5,304,614, 5,763,080, 5,795,946, 5,741,563, 6,156,856, 6,476,172, WO 97/19991, WO 00/01766, and WO 02/083753. See also, U.S. Pat. No. 5,504,172.
A very well known method of broadening composition distribution to enhance processing characteristics, is to use two different metallocene compounds during polymerization, and/or to use multiple reactors with varying polymerization conditions in each.
Another approach to improving the processability and/or strength of propylene polymers is to induce branching. This can be accomplished using diene monomer as described in U.S. Application Nos. 20010007896 and 20020013440, or by varying the type of metallocene used.
This invention provides a method for preparing unique propylene polymers using a single species of metallocene catalyst that, in a single reactor, can surprisingly produce propylene copolymers having a broad composition distribution, specifically increasing comonomer content with increasing molecular weight.